FINAL DRAFT UGANDA 
STANDARD  FDUS 861
First Edition
2009-mm-dd
 
Reference numbe  
r
FDUS 861: 2009
 ©UNBS 2009 
  
Method of test for evaluating absorptive solar receiver 
materials when exposed to conditions simulating stagnation in solar collectors with cover plates  
 FDUS 861: 2009 
ii © UNBS 2009 – All rights reserved
 Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS  2009  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilised in any 
form or by any means, electronic or mechanical, in cluding photocopying and microfilm, without prior written 
permission from UNBS. 
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The Executive Director 
Uganda National Bureau of Standards 
P.O. Box 6329 
Kampala  
Uganda 
Tel: 256 41 4505995 
Fax: 256 41 4286 123 
E-mail: unbs@infocom.co.ug
Web: www.unbs.go.ug FDUS 861: 2009 
© UNBS 2009 – All rights reserved  iii
 Contents  
Foreword………………………………………………………………………………………………..…………iv 
1   Scope ....................................................................................................................................................1  
2    Normative references ..........................................................................................................................1  
3    Terms abd definitions .........................................................................................................................1  
4   Significance and use ...........................................................................................................................2  
5    Test apparatus .....................................................................................................................................2  
6    Test specimens ....................................................................................................................................4  
7    Procedure .............................................................................................................................................5  
8   Report ...................................................................................................................................................6  
 
 FDUS 861: 2009 
iv © UNBS 2009 – All rights reserved
 Foreword 
Uganda National Bureau of Standards (UNBS) is a parastatal under the Ministry  of Tourism, Trade and 
Industry established under Cap 323, of the Laws of Uganda.  UNBS is mandated to co-ordinate the 
elaboration of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council.  
Committee membership 
The following organisations were represented on su bcommittee on Solar Energy, SC 3 under the Energy 
management standards Technical Committee, UNBS/ TC 13 in the development of this standard: 
• Battery Masters (U) Limited 
• Equator-sun (U) Limited 
• Incafex Solar Systems 
• Makerere University 
• Renewable Energy Department, Minist ry of Energy and Mineral Development 
• Solar Masters 
• Sonnerkraft Solar Systems 
• Uganda Institute of Professional Engineers 
• Uganda National Bureau of Standards 
• Uganda National Plumbers Association 
• Ultratec (U) Limited 
 
 
 FINAL DRAFT UGANDA STANDARD FDUS 861: 2009
 
© UNBS 2009– All rights reserved 1
 Method of test for evaluating absorptive solar receiver materials 
when exposed to conditions simulating stagnation in solar collectors with cover plates  
1 Scope 
1.1 This practice covers a test procedure for evaluatin g absorptive solar receiver materials and coatings 
when exposed to sunlight under cover plate(s) for long durations. This practice is intended to evaluate the 
exposure resistance of absorber materials and coatings  used in flat-plate collectors where maximum non-
operational stagnation temperatures  will be approximately 200 °C. 
1.2 This practice shall not apply to receiver materials us ed in solar collectors without covers (unglazed) or 
in evacuated collectors, that is, those that use a vacuum to suppres s convective and conductive thermal 
losses. 
1.3 The values stated in SI units are to be regarded as the standard. 
1.4 This standard does not purport to address all of the sa fety concerns, if any, associated with its use. It is 
the responsibility of the user of this standard to  establish appropriate safety and health practices and 
determine the applicability of regulatory limitations prior to use. 
2 Normative references 
The following referenced documents are indispensable for the application of this standard. For dated 
references, only the edition cited applies. For undated references, the latest edition of the referenced 
document (including any amendments) applies. 
FDUS ISO 9488, Solar energy — Vocabulary 
ISO 9060, Solar energy — Specification and classification of  instruments for measuring hemispherical solar 
and direct solar radiation  
ISO 9845-1, Solar energy — Reference solar spectral irradiance at the ground at different receiving conditions 
— Part 1: Direct normal and hemispherical solar irradiance for air mass 1.5  
ISO 10526, CIE standard illuminants for colorimetry 
ISO 15368, Optics and optical instruments — Measurement of reflectance of plane surfaces and 
transmittance of plane parallel elements  
3 Terms and definitions 
For purposes of this Uganda Standard, the following terms and definitions given in FDUS ISO 9488 shall 
apply. FDUS 861: 2009 
2 © UNBS 2009 – All rights reserved
 4 Significance and use 
4.1 Although this practice is intended for evaluating solar absorber materials and coatings used in flat-plate 
collectors, no single procedure can duplicate the wide r ange of temperatures and environmental conditions to 
which these materials may be exposed during in-service conditions. 
4.2 This practice is intended as a screening test for absorber materials and coatings. All conditions are 
chosen to be representative of those encountered in solar collectors with single cover plates and with no 
added means of limiting the temper ature during stagnation conditions. 
4.3 This practice uses exposure in a simulated collector  with a single cover plate. Although collectors with 
additional cover plates will produce higher temperatures at stagnation, this procedure is considered to provide 
adequate thermal testing for most applications. 
NOTE Mathematical modeling has shown th at a selective absorber, single glazed flat-plate solar collector can attain 
absorber plate stagnation tem peratures as high as 226 °C with an ambient te mperature of 37.8 °C and zero wind velocity; 
and a double glazed one as high as 245 °C under these conditions. The same configuration solar collector with a 
nonselective absorber can attain absorber stagnation temperatures as high as 146 °C, if single glazed, and 185 °C, if double glazed, with the same environmental conditions. 
4.4 This practice evaluates the thermal stability of abs orber materials. It does not evaluate the moisture 
stability of absorber materials used in act ual solar collectors  exposed outdoors. 
Moisture intrusion into solar collectors is a frequent oc currence in addition to condensation caused by diurnal 
breathing. 
4.5 This practice differentiates between the testing of spectrally selective absorbers and nonselective 
absorbers. 
4.5.1  Spectrally selective solar absorptive coatings and materials require testing in a covered enclosure that 
contains a selectively coated sample mounting plate, su ch that the enclosure and mounting plate simulate the 
temperature conditions of a selective flat-plate  collector exposed under stagnation conditions. 
4.5.2 Spectrally nonselective solar absorptive coatings and materials require testing in a covered enclosure 
that contains a nonselective coated sample mounting plate, such that the enclosure and mounting plate 
simulate the temperature conditions of a covered, nonse lective flat-plate collector exposed under stagnation 
conditions. 
5 Test apparatus 
5.1 Test enclosure 
As seen in Figure 1, the test enclosure shall consist of a box that approximates the dimensions of a typical 
flat-plate solar collector and shall have minimum dimensio ns of 0.75 m by 1.5 m by 0.1 m (29 in by 60 in by 4 
in.) deep. The box should be constructed of materials that are impervious to moisture. Wood should not be 
used for construction of the box. Care shall be taken to prevent water leakage at joints, seams, and seals. 
Prior to use, the test apparatus shall be placed in an operational environment  (pre-exposure) where all 
components are allowed to equilibrate at the stagnation temperature for a sufficient length of time to allow for outgassing of the components. This procedure may aid in eliminating contamination of the cover plate and the 
samples during actual testing periods and is especi ally important where coatings employing organic 
components are used. If the cover plate is in place duri ng this outgassing procedure, it shall be cleaned before 
the box is put into service in order to restore its original transmittance. FDUS 861: 2009 
© UNBS 2009 – All rights reserved  3
 5.2 Cover plate 
5.2.1 The box shall have a single cover plate that is gl azed and hinged to provide access. Two types of 
cover plate materials may be used: 
a) Type I - tempered low-iron glass with spectral c haracteristics approximating those shown in Figure 2; 
and 
b) Type II - alternative types of so lar transmitting glass or plastic materials might be used for the cover 
plate if the absorber is to be used under that material. 
5.2.2 The solar-weighted transmittance valu es of the cover plate test patches (5.2.3 and 5.2.4) shall remain 
above the indicated percentage of their initial values in the following wavelength regions: 
300 nm to 400 nm 90 % 
400 nm to 2 100 nm 95 %  
5.2.3 An easily removable test patch of the cover ma terial measuring 50 mm by 50 mm shall be fastened 
onto the inner surface of the transparent cover plate in  or near one lower corner. By periodically measuring 
the transmittance of this test patch, an indication of  the effect of any condensable effluents on the cover 
material can be monitored. 
5.2.4 An easily removable specimen of the cover plate ma terial measuring 50 mm by 50 mm should also be 
mounted directly on an exterior upper corner of the cover plate to monitor the effects of atmospheric 
contamination and ultraviolet degradation. These effects are generally more severe for plastic materials than 
for glass. 
5.3 Seals 
A seal that does not outgas at the stagnation temperat ure should be used to make the box weather-resistant. 
5.4 Insulation 
The bottom and sides of the enclosure shall be insulat ed to have a thermal conductance of less than 0.515 
W/(m2-K), that is, an R value of 11 or greater with materi als that do not outgas at the stagnation temperature. 
5.5 Sample mounting plate 
5.5.1 A metallic mounting plate with lateral dimensions a pproximately the same as the internal enclosure 
dimensions (less the thickness of the insulation on t he sides of the box) shall be mounted approximately 10 
mm above the bottom insulation by a thermally insulating material. 
5.5.2 The mounting plate used to support selective spec imens shall have a selective surface. The solar 
absorptance (a) of the selective co ating shall be greater than 0.90, and the room temperature emittance (e) 
shall be less than 0.15 at all times. 
5.5.2 The mounting plate used to support nonselective specimens shall be coated with any nonselective 
black coating that is ther mally resistant to temperatures up to ap proximately 200 °C. The solar absorptance 
(a) of the coatings (solar-weighted average from 350 nm to 2500 nm) shall be greater than 0.90 at all times. 
5.5.4 A specimen of the mounting plate material (50 mm  by 50 mm) shall be securely fastened to and in 
contact with the mounting plate. The purpose of this spec imen is to monitor the optic al characteristics of the 
mounting plate.  FDUS 861: 2009 
4 © UNBS 2009 – All rights reserved
 
 
Figure 1 – Typical cross-section of exposure test apparatus 
 
Figure 2 – Transmittance of low-iron glass 
5.6 Condensation control 
It is desirable to minimize or eliminate the presence of condensate on the inside of the cover plate and on the 
specimens during periods of solar irradiation to ensure  uniform results. The use of a desiccant may aid in 
minimizing condensation. In some geographical locations, weep holes drilled at the lower corners of the test 
enclosure may be required to drain condensate and thereby preclude the condensation of moisture on the cover plate. Weep holes shall be limited  in size and number so as not to disturb the equilibrium temperature of 
the test enclosure interior.  
6 Test specimens 
6.1 Test specimens shall be defined to be either the coati ng applied to a specific substrate or the absorber 
material itself (for materials other than coatings). FDUS 861: 2009 
© UNBS 2009 – All rights reserved  5
 6.2 The specimens shall be prepared in accordance wi th the procedures and conditions recommended by 
the coating or material supplier. 
7 Procedure 
7.1 Number of test specimens 
The number of test specimens shall be defined and se lected based on the need for replication and the test 
plan option used as described in 7.1.1 and 7.1.2. 
NOTE While replication is desirable whenever available and resources permit, a high level of experience with the 
weathering characteristics of any given product ma y permit single specimen testing with good reliability. 
For the case of Test Plan Option A, remove test specim ens at pre-selected deadlines (or remove sections by 
cutting, stamping, or otherwise segmenting), measure n ondestructively, and return to the test for continued 
exposure. 
For the case of Test Plan Option B, replicate specimens to permit withdrawals at preselected intervals without 
replacement. 
Exercise caution with test specimens that may outgas or are subjected to rapid initial decomposition at 
elevated temperatures. Such outgassing may significantly alter the transmittance of the cover plate. If this 
occurs, clean the cover plate (see 5.1) prior to resu ming testing. The decomposing test specimens may be 
removed from the test apparatus. Note observa tions and actions taken in the test report. 
7.2 Mounting test specimens 
Mount the test specimens on the sample mounting plate. Perform mounting and handling while wearing 
gloves so as not to damage or cont aminate the surface of either the te st material or the backing plate (see 
Note). The location and number of test specimens sha ll be consistent with thermal uniformity. Shading of 
specimens shall not occur, except just  after sunrise and just before sunset. 
NOTE The potential for galvanic corrosion should be considered when mounting the specimens. 
7.3 Orientation of test fixture and duration of exposure 
7.3.1 Mount the test fixture facing the equator with its major axis oriented east-west such that the angle 
between the normal to the plane of the test apparatus and the zenith is equal to the latitude of the testing site. 
7.3.2 Expose the specimens to a minimum total solar irradiation of 6.2 × 109 J/m2 (see Note). In order to 
develop kinetic or rate information, it is recommende d that the parameters chosen for studying these effects 
be measured at progressively longer exposure increments. 
NOTE This exposure value is based on the solar radiati on received in a 12-month period assuming an average value 
of 1.7 × 107 J/m2 per day.  
7.4 Measurements of test specimen properties 
Prior to exposure and at preselected exposure intervals, measure the prop erties of the test specimens as 
described in 7.4.1,  7.4.2 and 7.4.3. 
7.4.1 Solar absorptance 
Measure the solar absorptance in accordance with ISO 15368, unless otherwise specified. 
NOTE The spectral reflectance curves from which solar absorptance is calculated are often a more sensitive indicator 
of the onset of degradation of absorber materials than integr ated solar absorptance values. This is especially true for  FDUS 861: 2009 
6 © UNBS 2009 – All rights reserved
 changes occurring in spectral regions wher e there is a limited amount of  energy in the solar spectrum, that is, in the near 
infrared region. 
7.4.2 Emittance 
Measure the total hemispherical or norm al emittance, or both, using instruments in accordance with ISO 9060, 
unless otherwise specified. 
7.4.3 General appearance 
Assess the general appearance of the specimens in  accordance with ISO 10526, where applicable. 
7.5 Temperature measurements 
Daily measure and record the temperature of the abso rber sample mounting plate near solar noon. Measure 
the temperature at two locations, one located 150 mm from the upper edge and the other at the same 
distance from the lower edge along the minor axis of the plate (see 7.3. 1). Thermal sensors shall be in good 
thermal contact with the plate and shielded from direct solar radiation. 
7.6 Solar radiation measurements 
Measure total incident radiation on the plane containing the test fixture during the exposure period using a 
sensor meeting the requirements of a World Mete orological Organization second-class pyranometer. 
7.7 Other measurements 
7.7.1 Spectral transmittance 
Measure the spectral transmittance of the cover plate test patches periodically in the wavelength region 300 to 
2100 nm in accordance with ISO 15368 (see 5.2.2 and 5. 2.3). Measure plastic cover materials every three 
months. Measure glass cover materials annually. 
7.7.2 Solar absorptance 
Measure the solar absorptance of the test patch of the mounting plate ab sorber coating every three months 
(see 5.5.4). If a selective absorber is used, measure the infrared emittance every three months. Methods of 
measurement shall be as specified in 7.4. 
7.7.3 Appearance 
Visually inspect test specimens at least once pe r month, noting any changes in their appearance. 
7.8 Maintenance of test fixture 
Inspect the test fixture periodically and replace de fective components whose characteristics have degraded 
beyond the stated limits. 
Clean the cover plate as needed to maintain it s spectral transmittance above the stated limits.  
8 Report 
The report shall include the following: 
a) complete identification of the absorber material tested; FDUS 861: 2009 
© UNBS 2009 – All rights reserved  7
 b) complete identification of the substrate(s) us ed for absorber coatings, including composition and 
thickness; 
c) method of preparing the test specimens and any conditioning treatment; 
d) complete description of the test apparatus; 
e) location and dates of exposure; 
f) total incident solar irradiation received by the te st apparatus and documentat ion of the instrumentation 
and measurement procedure; 
g) type of cover plate material used, the spectr al transmittance measured initially and periodically 
throughout exposure, and documentation of the in strumentation and measurement procedure. Any 
cleaning or replacement of the cover plate shall be noted; 
h) maximum weekly temperature as recorded in 7.5; 
i) number and size of specimens tested; 
j) test specimen properties as measured in 7.4 and test patch properties as measured in 7.7; 
k) documentation of the instrument and the procedures used for measuring the reflectance and 
emittance of the test specimens and the solar spectral distribution used for computation of the optical 
properties; 
l) other observations such as those described in 7.1 and 7.7.3; and 
m) any change in the appearance or co ndition (fading, blistering, rusting, etc.) of individual specimens or 
group of specimens and identification of specimens that caused visible fogging of the cover plate and time intervals during which fogging occurred. 
 
 
 
 
 
 FDUS 861: 2009 
8 © UNBS 2009 – All rights reserved
 Bibliography 
 [1] ISO 10526, CIE Standard illuminants for colorimetry 
 [2] ISO 15368, Optics and optical instruments – Me asurement of reflectance of plane surfaces and 
transmittance of plane parallel elements  
UNBS All rights reserved  
 Certification marking 
Products that conform to Uganda standards may be  marked with Uganda National Bureau of Standards 
(UNBS) Certification Mark shown in the figure below.  
The use of the UNBS Certification Mark is gove rned by the Standards Act, and the Regulations made 
thereunder. This mark can be used only by those licens ed under the certification mark scheme operated by 
the Uganda National Bureau of Standards and in co njunction with the relevant Uganda Standard. The 
presence of this mark on a product or in relation to a product is an assurance that the goods comply with the 
requirements of that standard under a system of supervision, control and  testing in accordance with the 
certification mark scheme of the Uganda National  Bureau of Standards. UNBS marked products are 
continually checked by UNBS for conformity to that standard.  
Further particulars of the terms and conditions of  licensing may be obtained from the Director, Uganda 
National Bureau of Standards.  
 
 
2009 – DUS 
 
ICS 885: 2009 
27.160  
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